A conventional internal combustion engine has for each cylinder, one or two intake valves and one or two exhaust valves. In the normal combustion cycle, the intake valves open during the intake phase, the exhaust valves open during the exhaust phase and both sets of valves remain closed during the compression and combustion phases.
During the intake phase when the intake valve opens, the negative pressure in the combustion chamber created by the downward motion of the piston draws intake air from the intake manifold into the combustion chamber. Ideally, when the pressure in the intake manifold and the combustion chamber equalizes, the intake valves close to keep the maximum charge in the combustion chamber. However, after the intake valve for a particular cylinder closes the air in the intake manifold becomes stagnant until the beginning of the intake cycle for the next cylinder. In the initial stages of the intake cycle for the next cylinder, the intake valve opens and the stagnant air is initially drawn into the combustion chamber by exhaust scavenging during a brief time in the intake phase during which both the intake and exhaust valves are open. Then, after the exhaust valve is closed, the negative pressure effect takes over to bring the remainder of the charge into the combustion chamber via the intake valve. This overlap feature which is designed to create more volumetric efficiency at higher RPMs allows some raw fuel to escape into the exhaust manifold creating unwanted exhaust emissions.
Additionally, the combustion cycle in a conventional engine causes severe heating to the intake and exhaust valves causing undesirable hot spots in the combustion chamber. As a result, a conventional engine is required to use a richer fuel to air ratio to control combustion temperatures within the fuel's flash point. Thus fuel is used not only as an energy source, but also to cool the combustion chamber.
Moreover, the rapid opening and closing of the intake and exhaust valves in a conventional engine leads to vibration and noise in the engine head.